Problem description:

You are given two non-empty linked lists representing two non-negative integers. The digits are stored in reverse order and each of their nodes contain a single digit. Add the two numbers and return it as a linked list

You may assume the two numbers do not contain any leading zero, except the number 0 itself.

Example:

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Input: (2 -> 4 -> 3) + (5 -> 6 -> 4)
Output: 7 -> 0 -> 8
Explanation: 342 + 465 = 807.

Solution:

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/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode() {}
 *     ListNode(int val) { this.val = val; }
 *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
 * }
 */
class Solution {
    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
        ListNode head = new ListNode(0);
        ListNode dummy = head;
        int carry = 0;
        while(l1 != null || l2 != null || carry != 0){ 
            if(l1 != null){
                carry += l1.val;
                l1 = l1.next;
            }
            if(l2 != null){
                carry += l2.val;
                l2 = l2.next;
            }
            
            dummy.next = new ListNode(carry % 10);
            carry /= 10;
            dummy = dummy.next;
        }
        return head.next;
    }
}

time complexity: $O(n)$
space complexity: $O(1)$, if not consider result space.
reference:

related question:
43. Multiply Strings
445. Add Two Numbers II